i3bar: Group child processes for signalling

Patch status: merged

Patch by Tony Crisci

Long description:

Set the process group id of the child process by calling `setpgid` after
forking and before calling `exec`.
The process group ID will be set to the process ID of the forked
process. Processes spawned by this child process will also have this
group ID.
Send signals to the process group with `killpg`. This will send the
signal to all of the process group.
fixes #1128